40
$\begingroup$

Wind information can be reported by various sources (ATIS, METAR, TAF, spoken on the radio, etc). I was taught that officially some of these sources are relative to magnetic north and others to true north (although practically this isn't always followed). Which sources are supposed to use which reference system?

$\endgroup$

4 Answers 4

47
$\begingroup$

The general rule is:

If you read it, it's true. If you hear it, it's magnetic.

All charts and textual sources (METAR, TAF, winds aloft, surface analysis charts, etc) use true north as the reference.

ATIS/AWOS/ASOS broadcasts, or any information a controller gives you over the radio, is magnetic.

Wind direction broadcast over FAA radios is in reference to magnetic north.

AIM Section 7-1-11 (page 7-1-26 in the 5/26/16 edition)

One exception to the "if you hear it" rule is that a FSS briefer will read you the winds referenced to true north, since they're just reading you the charts/textual information.

(This is at least true in the United States... other countries may vary in some instances)

$\endgroup$
7
  • 8
    $\begingroup$ It's the same in Europe. Wind information communicated by the tower when a landing/takeoff clearance is given is in degrees magnetic. METAR/TAFs are in degrees true. Not that it matters much because magnetic variation is almost zero in Western Europe $\endgroup$ Dec 18, 2013 at 16:21
  • 3
    $\begingroup$ I believe one very important difference is TWEBs, or Transcribed Weather Broadcasts. Because they are an automated broadcast of a written report, they are reported in true headings, just like a written report, even though you technically hear it. $\endgroup$
    – abelenky
    May 13, 2016 at 20:07
  • $\begingroup$ Oops, I down-voted before I researched my answer completely. It appears we were both wrong. The AIM seems to indicate, as I expected, that the ASOS/AWOS data is the same as METAR, as I had expected. What I was wrong about them both being magnetic. $\endgroup$ Apr 4, 2017 at 19:33
  • $\begingroup$ @2NinerRomeo you might want to research your answer... a little more completely ;) $\endgroup$ Apr 6, 2017 at 3:53
  • 1
    $\begingroup$ Sorry for the necro but I believe that the D-ATIS (digital ATIS) is also an exception to this rule. It also reports wind direction relative to magnetic north. It is the textual version of the ATIS that is shown in the cockpit and basically serves the same function as a spoken ATIS so that makes sense. $\endgroup$
    – Jupiter
    Sep 5, 2018 at 20:42
0
$\begingroup$

In the United States, the principal guide to weather products is AC 00-45G "Aviation Weather Services". As of this writing it appears Change 1 is current, published July 2010. Citations given here are to that revision. The Airman's information manual has information on AWOS and ASOS.

Here are some examples:

  • METAR: True 3.1.3.5
  • AWOS & ASOS: True AIM 7-1-11 Figure 7-1-5
  • TAF: True 7.2.2.5
  • WA (Winds Aloft): True 7.4.2
  • UA (Pilot Reports): Magnetic 3.2.1.9 (I'd suppose you get whatever the pilot reports.)
  • Surface Analysis Chart: True 5.1.2.3.5

When calling flight service, the briefer will have available the same information products that you can reach on aviationweather.gov. I expect they will know the direction reference for the information they are giving you.

$\endgroup$
3
  • $\begingroup$ When you say AWOS/ASOS to a pilot, they think of the automated radio report, where wind is given as magnetic, not true. I think you're getting mixed up because the ASOS equipment is also used to create METARs and other textual products, where wind is reported as true. Quote from nws.noaa.gov/asos/pdfs/aum-toc.pdf (section 3.2): "Wind direction is reported relative to true north in the METAR/SPECI message.... Wind direction is reported relative to magnetic north in the computer-generated voice messages" $\endgroup$ Apr 6, 2017 at 3:37
  • $\begingroup$ Also, AC 00-45G is cancelled. AC 00-45H is current. $\endgroup$ Apr 6, 2017 at 3:39
  • $\begingroup$ AIM Figure 7-1-5 is about decoding a textual METAR generated from ASOS/AWOS. It has nothing to do with the verbal radio broadcasts. See the note on page 7-1-26: "Note- Wind direction broadcast over FAA radios is in reference to magnetic north." (taken from the 5/26/16 AIM). $\endgroup$ Apr 6, 2017 at 3:50
0
$\begingroup$

The way I remember it is that if it relates to an aerodrome its magnetic, because runway numbers are in magnetic and the pilot needs to know how the wind relates to the runway in use so the amount of headwind / crosswind can be assessed.

If it relates to en-route then its in true, because flight planning is done in true (with conversions to the correct magnetic heading to set on the compass at each step).

$\endgroup$
1
  • $\begingroup$ This is what I thought until I looked it up. I was gravely disappointed because this is how I thought it should be. $\endgroup$ Apr 5, 2017 at 15:00
-2
$\begingroup$

Another good general rule

Everything the weather man tells you it's true ... the rest is magnetic

$\endgroup$
5
  • $\begingroup$ This seems opposite to the previous answer. Are you talking about the TV weather man? That doesn't seem useful for aviators. $\endgroup$
    – PJNoes
    Mar 16, 2016 at 15:58
  • $\begingroup$ @PJNoes How did you get TV weather from this? It's an attempt to memorize that weather information is provided in True North, while operational (atis, tower) is Magnetic. "True" in this case meaning True North and not Accurate, Correct,etc.. hence the funny pun... well, attempt of, anyway... $\endgroup$
    – Radu094
    Mar 17, 2016 at 12:37
  • $\begingroup$ I grew up when the only weather was from the weather man on TV. I got the funny pun, but the previous answer is easier for me to remember. No offence meant. $\endgroup$
    – PJNoes
    Mar 17, 2016 at 19:10
  • $\begingroup$ If the weatherman wrote it, its true. If he spoke it or tells it, its magnetic. Written is True. $\endgroup$
    – abelenky
    May 13, 2016 at 20:02
  • $\begingroup$ when is the weatherman speaking to you using magnetic north ? ... is what I'd like to understand .... $\endgroup$
    – Radu094
    Nov 11, 2016 at 18:44

This site is temporarily in read-only mode and not accepting new answers.

Not the answer you're looking for? Browse other questions tagged .